Switched-mode is an emerging technology for achieving high power efficiency in Radio Frequency (RF) power amplifiers. Switched-mode means that the power transistors are either in a fully conducting state or in a completely shut-off state. A filter at the output is then often needed to remove unwanted harmonics and quantization noise of the switch frequency.
A method of achieving a band pass RF signal is to use band pass sigma delta modulation (BPSDM) working at four times the RF carrier frequency. Developments based on this method are disclosed for example in EP-A1-2330734 or EP-A1-2403136. However such techniques lead to the constraints of generating four pulses per carrier period and having the whole digital chain, including BPDSM, work at a very high frequency higher than the wanted RF carrier frequency, which causes huge technology constraints.
Craven P. “Toward the 24-bit DAC: novel noise-shaping topologies incorporating correction for the nonlinearity in a PWM output stage”, J. Audio Eng. Soc., Vol. 41, No 5. (XP000432100) discloses a nonlinear noise shaper, which includes a digital simulation of the intrinsic PWM non linearity and correction through feedback or a combination of feedforward and feedback.
WO-A1-2001/97384 discloses methods for reducing distortion and noise in high power digital PWM amplifiers by measuring the difference between the desired output signal and the actual output signal on a pulse by pulse basis.
WO-A2-9215153 discloses a signal convertor comprising a pulse modulator, and means for modifying the signal input thereto in dependence upon the error in previous values of the output thereof, to reduce the effects of the error within a desired signal band.